State of the Art
Dextran is a polysaccharide formed of glucose units, the chain lengthening of which is catalyzed by dextran sucrase. The biosynthesis of dextran has been demonstrated in numerous bacteria, especially in Streptococcus mutans, Leuconostoc mesenteroides ssp. mesenteroides and Leuconostoc mesenteroides ssp. dextranicum. Leuconostoc produce the enzyme dextran sucrase and secrete it into the culture medium in the presence of sucrose. This enzyme, dextran sucrase, then synthesizes dextran from the sucrose substrate. Dextran has applications in several fields. It is used especially in biochemistry as a support for filtration chromatography on a gel of the Sephadex type. Additionally, in the field of therapeutics, it is used as a substitute for blood plasma (Biochimie generale (General Biochemistry)--J. H. WEIL--Masson, 6th edition--1990--p. 171).
Furthermore, dextran synthesized by a strain of Leuconostoc dextranicum is applied in the food industry for the texturing of food products such as yoghurts, cream desserts, milk-based drinks and salad dressings. European Patent Application Publication No. EP 0 363 633 demonstrates the synthesis of dextran by a strain of Leuconostoc dextranicum and in particular by the strain Leuconostoc dextranicum NRRL-B-18242. Additionally, that patent application describes especially a composition containing dextran synthesized by this bacterium and the use of this composition in the food sector. Moreover, the taxonomy of the bacterial strains of the genus Leuconostoc has been revised several times.
Garvie et al. (International Journal of Systematic Bacteriology, 118-119, 1983) describe the taxonomy of the bacterial strains of the genus Leuconostoc, established according to a criterion of homology in terms of the deoxyribonucleic acid (DNA). The bacteria, previously classed as Leuconostoc mesenteroides, Leuconostoc dextranicum and Leuconostoc cremoris, although of different phenotype, have a very high degree of homology in respect of their DNA. Therefore, according to this taxonomy, these bacteria are subspecies of Leuconostoc mesenteroides and are respectively called Leuconostoc mesenteroides ssp. mesenteroides, Leuconostoc mesenteroides ssp. dextranicum and Leuconostoc mesenteroides ssp. cremoris.
In addition, J. B. Milliere et al. (Journal of Applied Bacteriology, 67, 529-542, 1989) describe a taxonomic analysis performed on 81 strains of the genus Leuconostoc, including 11 strains of Leuconostoc mesenteroides ssp. cremoris. This analysis relies on the taxonomy established by Garvie et al. and is based on the following criteria in particular: the capacity of these strains to ferment various sugars, the capacity of these strains to utilize citrate and the capacity of these strains to produce dextran. The document notes the fact that a strain of Leuconostoc mesenteroides ssp. cremoris does not synthesize dextran. Also, such a strain is distinguished and defined by the fact that it does not ferment pentoses.
No strain of Leuconostoc mesenteroides ssp. cremoris has yet been isolated which is capable of synthesizing dextran. Now, Leuconostoc mesenteroides ssp. cremoris is of major importance in the manufacture of dairy products such as, for example, yoghurt-type fermented specialities or dairy creams. It would therefore be very valuable to be able to use such bacteria, capable of synthesizing dextran of pleasant texture and taste, especially for texturing this type of food product.
The object of the present invention is to meet this need.